Attivare un bottone

https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_active_element

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
/* Style the buttons */
.btn {
  border: none;
  outline: none;
  padding: 10px 16px;
  background-color: #f1f1f1;
  cursor: pointer;
  font-size: 18px;
}

/* Style the active class, and buttons on mouse-over */
.active, .btn:hover {
  background-color: #666;
  color: white;
}
</style>
</head>
<body>

<h1>Active Button</h1>
<p>Highlight the active/current (pressed) button.</p>
  
<div id="myDIV">
  <button class="btn">1</button>
  <button class="btn active">2</button>
  <button class="btn">3</button>
  <button class="btn">4</button>
  <button class="btn">5</button>
</div>

</body>
</html>

https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_active_element

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
/* Style the buttons */
.btn {
border: none;
outline: none;
padding: 10px 16px;
background-color: #f1f1f1;
cursor: pointer;
font-size: 18px;
}

/* Style the active class, and buttons on mouse-over */
.active, .btn:hover {
background-color: #666;
color: white;
}
</style>
</head>
<body>

<h1>Active Button</h1>
<p>Highlight the active/current (pressed) button.</p>

<div id="myDIV">
<button class="btn">1</button>
<button class="btn active">2</button>
<button class="btn">3</button>
<button class="btn">4</button>
<button class="btn">5</button>
</div>

<script>
// Add active class to the current button (highlight it)
var header = document.getElementById("myDIV");
var btns = header.getElementsByClassName("btn");
for (var i = 0; i < btns.length; i++) {
btns[i].addEventListener("click", function() {
var current = document.getElementsByClassName("active");
current[0].className = current[0].className.replace(" active", "");
this.className += " active";
});
}
</script>

</body>
</html>